Composite material based on a polymer matrix comprising fluoroelastomers or fluoroplastomer

ABSTRACT

Process for the production of a composite material based on a polymer matrix comprising homogeneously dispersed therein an organic and/or inorganic filler, said process consisting in mixing together the aqueous dispersions of the single components, in which the dispersed particles of the dispersion have a surface charge of the same sign and &#34;zeta potential&#34; between 10 and 100 mV (in absolute value) and wherein the ratio (in absolute value) between the &#34;zeta potentials&#34; of the particles of each dispersion is between 0.2 and 5.

RELATED APPLICATION

This is a division of 07/019,255, filed Feb. 26, 1987, now U.S. Pat. No.4,798,854, which is a continuation-in-part of copending application Ser.No. 848,278, filed Apr. 4, 1986, now abandoned.

THE DISCLOSURE

1. Field of the Invention

The present invention relates to a process for the production of acomposite material based on a polymeric matrix. More particularly, thepresent invention relates to a process for the preparation of acomposite material based on a polymer matrix comprising, homogeneouslydispersed therein, an organic and/or inorganic filler.

2. The Prior Art

As is known, in many applications there are used materials comprising apolymer matrix in which are dispersed organic and/or inorganic fillerswhich impart to the composite material special properties such as color,mechanical resistance, etc.

These composite materials in general are produced by mixing the organicand/or inorganic filler in the form of a powder with the polymer also inform of a powder and usually in the presence of a dispersing medium, forexample oil. This type of process requires that the polymer beprocessable on special machines, and that, moreover, the materials usedas fillers possess specific dispersibility characteristics.

In the case of thermoplastic polymers, there are required both hightemperatures as well as a high quantity of energy for the mixing of thefiller with the polymer in the molten state.

It is also known to produce composite materials by using a processconsisting in mixing an aqueous dispersion of the polymer (latex) andthe polymer in the form of a powder, and by then coagulating theresulting dispersion and then separating the coagulated compositematerial. Even if this process of mixing requires relatively smallquantities of energy, the composite material thus obtained does not showa satisfactory homogeneity as required in certain applications.

Moreover, the addition of the organic and/or inorganic filler to thelatex may lead to the coagulation of the polymer before a completehomogenization can be achieved.

THE PRESENT INVENTION

An object of the present invention is to provide a process for thepreparation of a composite material based on a polymer matrix containinghomogeneously dispersed therein an organic and/or inorganic filler, saidprocess not showing the above reported drawbacks.

More particularly, the object of the present invention is to provide acomposite material based on a polymer matrix, comprising dispersedtherein an organic and/or inorganic filler, showing a high grade ofuniformity, prepared by a quite simple process and with the use of onlysmall quantities of energy.

It has now, surprisingly, been found that these, and other objects areachieved only if the following conditions are satisfied:

(a) the polymer matrix and the organic and/or inorganic filler are mixedtogether in the form of an aqueous dispersion; and

(b) the particles of the dispersed system have a high degree ofdispersion and show a comparably high stability.

According to the present invention, said conditions are satisfied byusing a process for the production of a composite material based on apolymer matrix, comprising homogeneously dispersed therein an organicand/or inorganic filler, said process comprising mixing together thesingle components in the form of aqueous dispersions in which theparticles have a surface charge of the same sign and a "zeta potential"(in absolute value) between 10 mV and 100 mV and the ratio (in absolutevalue) between the "zeta potential" of the particles of each dispersionis between 0.2 and 5, and then by coagulating the mixed dispersion thusobtained.

"Zeta potentials" (in absolute value) between 15 and 70 mV and ratios(in absolute value) between "Zeta potentials" between 0.5 and 2 arepreferred in the process of the present invention.

In the preparation of the composite material, any polymer matrix may beused, provided that it be in the form of a latex or, preferably,obtained by emulsion polymerization.

Examples of polymer that may be used in the process of the presentinvention are:

fluorinated polymers and elastomers such as polytetrafluoroethylene;

copolymer of vinylidene fluoride and hexafluoropropylene;

terpolymers comprising vinylidene fluoride, hexafluoropropylene andtetrafluoroethylene terpolymers;

terpolymers of vinylidene fluoride, tetrafluoroethylene andperfluoroalkyl-vinyl-ether;

copolymers of tetrafluoroethylene and perfluoroalkyl-vinyl-ether;

copolymers of tetrafluoroethylene and propylene;

copolymers of vinylidene-fluoride and chlorotrifluoro-ethylene;

copolymers of vinylidenefluoride and hydropentafluoropropylene;

terpolymers of vinylidene fluoride, hydropentafluoropropylene andtetra-fluoroethylene; etc., and the corresponding products comprising inthe chain small quantities of trifluoro-bromoethylene;

chlorinated (co)polymers such as polyvinylchloride andpolyvinylidenechloride;

polymers and copolymers such as polyacrylonitrile, polymethacrylates,polymethylmethacrylates, etc.;

polybutadiene; polyisoprene; polycarbonates;

acrylonitrile-butadiene-styrene polymers, known as ABS resins;

vinyl (co)polymers such as polyvinylacetate, ethylene-vinylacetate (EVA)copolymers, polyvinyl-alcohol (PVA), etc.,

and in general any polymer and copolymer obtained by emulsionpolymerization.

Examples of inorganic fillers that may be used in the preparation of thecomposite materials are: pigments, graphite, bronze in powder form,ground glass in the form of spheres or fibers, molybdenum sulphide,silica, steel powder, metal oxides, carbonates, silicates, etc.

As organic fillers may be cited organic pigments, stabilizers, anypolymer or resin different from that which forms the polymer matrix,etc.

According to the present invention, composite materials in which thecharge (or filler) is homogeneously dispersed in the matrix, areobtained only if the single components are mixed together starting frompreformed dispersions in which the particles are completely wetted bythe dispersing medium and which show a surface charge of the same signand comparable stabilities. By the term "stability" is meant thecapacity of the particles to remain in the dispersed state.

The measure of the stability of a dispersed system may be carried out bythe "Zeta potential", defined as the electric potential at theparticle-solution plane of slippage. The "zeta potential" of particlesin a dispersion may be determined by means of an electrokinetic measuresuch as for instance: electrophoresis, electro-osmosis,potential/current flow, etc., as described for example in "ColloidScience", Vol. 1, by H. R. Kruyt, published by Elsevier Publishing Co.,in 1952.

The adjusting of the "zeta potential" value of the components of eachdispersion may be carried out acting on the mechanisms which control theformation of the surface charge. A suitable method for varying the "zetapotential" is that of varying the pH of the dispersion or otherwise byadding surfactants, polyelectrolytes or ions specifically absorbed, suchas for example: sodium laurylsulphate, ammonium perfluorooctanoate,sodium polyacrylate, sodium hexametaphosphate, sodium phosphate, etc.

After the mixing of the dispersions of the single components, theresulting dispersion is coagulated. The coagulation phase is preferablycarried out very quickly in order to avoid the selective coagulation ofthe single components of the dispersion, with consequentialdishomogeneity of the coagulate.

Any technique for coagulating the dispersion may be used, such as forinstance: vigorous stirring, the addition of electrolytes or theaddition of a liquid in which the dispersions are not stable, or acombination of these techniques.

The preferred method in the process of the present invention is theaddition of electrolytes such as sulphuric acid, magnesium nitrate,aluminum sulphate, etc., at a concentration at least sufficient tocoagulate the dispersion, combined, if necessary, with a vigorousstirring.

The separation of the coagulate from the dispersing medium may beeffected by using conventional techniques such as flotation, filtration,centrifugation, decantation, or a combination of these techniques.

The use of the various separation techniques depends on the polymericmatrix used. More particularly, when the polymer phase ispolytetrafluoroethylene, a preferred method may be the flotation of thecoagulate by using vigorous stirring, while for composite materialsbased on polyvinylchloride or on tetrafluoroethylene-vinylidene fluoridecopolymer, it is advisable to use separation by filtration.

In order to avoid possible negative influences due to the presence ofelectrolytes in the coagulate, after the separation there may be carriedout one or more washings of the coagulate.

The composite materials thus obtained according to the process of thisinvention are perfectly uniform upon examination under the microscope.

Some of these composite materials are new. One of these new compositematerials consists essentially of a fluoroelastomer matrix containing afluoroplastomer in a dispersed state, said dispersed fluoroplastomerbeing substantially dispersed in the form of the primary particles, boththe fluoroelastomer matrix and the dispersed fluoroplastomer beingobtained by emulsion or dispersion polymerization in an aqueous phase.Commonly, at least 90% of the fluoroplastomer particles are present asprimary particles. The primary particles of polymers are, as is wellknown, the polymer particles as they are obtained at the end of thepolymerization.

The dispersed fluoroplastomer amounts are commonly from 0.1 to 50% byweight of the composite material, and preferably from 5 to 50% byweight.

Another of these new composite materials consists essentially of afluoroplastomer matrix containing a fluoroelastomer in a dispersedstate, said fluoroelastomer being uniformly distributed inside thecoagulated flocks of the fluoroplastomer, both the fluoroplastomermatrix and the dispersed fluoroelastomer being obtained by emulsion ordispersion polymerization in an aqueous phase.

The dispersed fluoroelastomer amounts are commonly from 0.1 to 50% byweight of the composite material, and preferably from 5 to 40% byweight.

The preferred fluoroelastomers, used either as matrix or as dispersedphase, are those already cited hereinbefore, i.e., copolymers ofvinylidene fluoride and hexafluoropropylene and, optionallytetrafluoroethylene; terpolymers of vinylidene fluoride,tetrafluoroethylene and a perfluoroalkylvinyl ether; copolymers ofvinylidene fluoride and hydropentafluoropropylene and, optionally,tetrafluoroethylene and the corresponding copolymers containing in thechain a small quantity of trifluorobromoethylene.

The most preferred fluoroelastomers are the copolymers of vinylidenefluoride, hexafluoropropylene and, optionally, tetrafluoroethylene,e.g., a copolymer containing from 53 to 80% by moles of vinylidenefluoride, from 19 to 22% of hexafluoropropylene, and from 0 to 25% oftetrafluoroethylene.

The preferred fluoroplastomers, either used as matrix or as dispersedphase, are those already cited hereinbefore, i.e.,polytetrafluoroethylene; copolymers of tetrafluoroethylene and aperfluoroalkyl vinyl ether; copolymers of vinylidene fluoride andchlorotrifluoroethylene; and copolymers of tetrafluoroethylene andpropylene.

The most preferred fluoroplastomer is polytetrafluoroethylene.

EXAMPLES

In order still better to understand the present invention the followingexamples are given. However, they are not to be taken as in any waylimitative of the inventive scope of the invention. In the examples, allthe percentages and the parts are by weight unless otherwise indicated.

EXAMPLE 1

A dispersion of a red pigment based on iron oxide (Bayer 110M R) wasprepared in distilled water, at a concentration of 10 g/l, by means ofultrasonics.

The particles of dispersed pigment showed a "zeta potential" equal to-25 mV.

Thereupon, 20 ml of the dispersion were added under a mild stirring (50r.p.m.) to a polytetrafluoroethylene latex (PTFE) containing 40 g ofpolymers.

The thus-obtained dispersion was then diluted with distilled water untila concentration of the PTFE equal to 100 g/l was obtained.

In the latex the PTFE particles showed a "zeta potential" of -30 mV,measured by microelectrophoresis.

To this mixed dispersion thus obtained there was added 10 ml of H₂ SO₄(30% by weight) while the stirring speed was rapidly brought up to 700r.p.m.

The dispersion coagulated completely in a time below 5 minutes. Thestirring was maintained for another 10 minutes in order to float thecoagulate.

The powder, separated from the liquid by filtration on a nylon net, wasthen dried for 12 hours at 140° C.

A thin ribbon prepared with the pigmented product showed a perfectlyuniform color, as well as mechanical and dielectrical properties similarto those of non-pigmented ribbon, and the absence of pigment coagulatesunder microscopic analysis.

EXAMPLE 2

A pigment based on cadmium sulphoselenide (CP 2400, Ferro Co., USA R) isdispersed, at a concentration of 10 g/l, in an aqueous solutioncontaining 1 g/l of sodium hexametaphosphate, by using a ball mill ofthe "Red Devil" type.

The pigment particles have a "zeta potential" of -46 mV, measured byelectromosis.

20 ml of the dispersion were then added, under mild stirring (50r.p.m.), to a PTFE latex containing 40 g of polymer; the dispersion wasthereupon diluted with distilled water to a concentration of 100 g/l.

In the latex the PTFE particles showed a "zeta potential" of -30 mV,measured by electrophoresis.

To the mixed dispersion were then rapidly added under constant stirring20 ml of a 1M solution of Mg(NO₃)₂ thereby causing bulk coagulation ofthe particles.

The coagulate was then separated by floating, increasing the stirringvelocity to 700 r.p.m. The powder showed a uniform color and was driedat 140° C. for 12 hours.

A ribbon prepared with the pigmented powder showed a uniform color, aswell as dielectric and mechanical properties similar to those of anon-pigmented ribbon, and showed absence of coagulated pigment undermicroscope analysis.

EXAMPLE 3

A sample of graphite (FOLIAC X6236 R) was dispersed at a concentrationof 10 g/l, in an aqueous solution containing 1 g/l of sodiumpolyacrylate, by using a "Red Devil" ball mill. 60 ml of the dispersionwere thereupon added to 120 ml of a PTFE latex containing 40 g ofpolymer.

The "zeta potential" of the graphite particles, measured byelectroosmosis, was -15 mV, while the "zeta potential" of the PTFE,measured by electrophoresis, amounted to -30 mV.

The dispersion thus obtained was then diluted, under mild stirring (50r.p.m.), up to a volume of 400 ml. Still under mild stirring, there werethen added 20 ml of a 1M solution of Mg(NO₃)₂ causing the completecoagulation of the slurry. The thus-coagulated polymer was then floatedunder stirring of about 700 r.p.m.

The obtained powder, dried at 140° C. for 12 hours, was perfectlyuniform under microscopic analysis. A thin ribbon prepared with thedried powder showed a uniform color.

EXAMPLE 4

40 ml of a slurry, obtained by dispersing 25 g of glass fiber in 100 mlof an aqueous solution containing 1 g/l of sodium hexametaphosphate,were added to 120 ml of a PTFE latex containing 40 g of the polymer. The"zeta potential" of the glass fiber, measured by electroosmosis, was -51mV, while the "zeta potential" of the PTFE, measured by electrophoresis,was -30 mV.

The obtained slurry, diluted to a volume of 400 ml, was then additionedwith 20 ml of a 1M solution of Mg(NO₃)₂ under mild stirring (100r.p.m.). The addition of the electrolyte caused the complete coagulationof the dispersion. The coagulate was thereupon floated, under vigorousstirring (700 r.p.m.), for a period of 15 minutes.

The powder thus obtained, dried for 12 hours at 140° C., undermicroscopic analysis was shown to be formed of glass fibers individuallydispersed in the polymer matrix.

EXAMPLE 5

A PTFE latex was added, in an amount of 10% by weight, to a latex madeof a Tecnoflon fluoroelastomer (registered Montedison trademark). Thisfluoroelastomer contains 80% by moles of vinylidene fluoride and 20% ofhexafluoropropylene. The "zeta potential" of the PTFE latex was -30 mV,while the "zeta potential" of the fluoroelastomer was -50 mV, bothmeasured by electrophoresis.

Then 200 ml of the mixed latex were slowly added to an equal volume ofan aqueous solution containing 8 g/l of Al₂ (SO₄)₃, maintained undervigorous stirring (500 rpm). The slurry coagulated instantly.

The coagulate, after separation by filtration and dried for 12 hours at60° C., appeared under the microscope to be perfectly uniform.

EXAMPLE 6

A PTFE latex, containing 40 g of polymer, was additioned with adispersion of red pigment CP2400R (by Ferro Co., USA) and a TiO₂dispersion. The total amount of added pigments was equal to 3.5% basedon the PTFE.

The "zeta potentials" of the particles are:

--for the PTFE=-30 mV

--for the red pigment=-46 mV

--for the TiO₂ pigment=-40 mV.

There were prepared various dispersions, with ratios by weight of redpigment/TiO₂ pigment of 10:1, 2.5:1, 1:1, 1:2.5, 1:5, 1:10 and 1:20.

The dispersions were coagulated as described in Example 2. The powders,dried for 12 hours at 140° C., were uniformly colored.

The color of the tablets, obtained by sintering the powders at 400° C.,ranges uniformly from red to pink as the red pigment/TiO₂ pigment ratiogoes down.

EXAMPLE 7

66.7 ml of a polyvinylchloride latex (at a 300 g/l concentration), and10 ml of dispersion of a yellow F 897 pigment R (Ferro Co., USA) (at aconcentration of 10 g/l) were mixed with an aqueous solution containing0.1 g/l of sodium hexametaphosphate, and 83.4 ml of distilled H₂ O.

The "zeta potentials" of the particles, measured by electrophoresis,were:

--for the polyvinyl chloride: -35 mV,

--for pigment: -25 mV.

Thereupon, under a mild stirring, there were admixed 20 ml of a 1Msolution of Mg(NO₃)₂, thereby causing the complete coagulation of theslurry.

The colored coagulate was then separated by filtration and dried for 48hours at 50° C. The resulting powder was uniformly colored.

EXAMPLE 8

3 liters of a latex containing 113 g of polytetrafluoroethylene areadded to 2 liters of a latex containing 350 g/l of the samefluoroelastomer as in Example 5.

The zeta potential of the first latex is -40 mV; the zeta potential ofthe second is -50 mV, both measured by electrophoresis.

The mixed latex is added slowly to an equal volume of an aqueoussolution containing 6 g/l of aluminum sulfate, maintained under vigorousstirring. The slurry coagulates instantly.

The coagulate, separated by filtration and dried at 60° C. overnight,contains 33% by weight of polytetrafluoroethylene and 67% offluoroelastomer.

Examination of the coagulated particles by scanning electron microscopyshows that the PTFE particles are dispersed inside the continuous matrixof fluoroelastomer, essentially in the form of primary particles.

EXAMPLE 9

3.85 liters of a latex containing 113 g/l of polytetrafluoroethylene areadded to 1.25 liters of a latex containing 350 g/l of the samefluoroelastomer as in Example 5.

The zeta potential of the first latex is -40 mV; the zeta potential ofthe second is -50 mV, both measured by electrophoresis.

The mixed latex is added slowly to an equal volume of an aqueoussolution containing 6 g/l of aluminum sulfate, maintained under vigorousstirring. The slurry coagulates instantly.

The coagulate, separated by filtration and dried at 60° C. overnight,contains 50% by weight of polytetrafluoroethylene and 50% offluoroelastomer.

Examination of the coagulated particles by scanning electron microscopyshows that the PTFE particles are dispersed inside the continuous matrixof fluoroelastomer, essentially in the form of primary particles. 95% ofthe PTFE particles are dispersed as primary particles.

EXAMPLE 10

1.35 liters of a latex containing 113 g/l of polytetrafluoroethylene areadded to 3.9 liters of a latex containing 350 g/l of the samefluoroelastomer as in Example 5.

The zeta potential of the first latex is -40 mV; the zeta potential ofthe second is -50 mV, both measured by electrophoresis.

The mixed latex is added slowly to an equal volume of an aqueoussolution containing 6 g/l of aluminum sulfate, maintained under vigorousstirring. The slurry coagulates instantly.

The coagulate, separated by filtration and dried at 60° C. overnight,contains 10% by weight of polytetrafluoroethylene and 90% offluoroelastomer.

Examination of the coagulated particles by scanning electron microscopyshows that the PTFE particles are dispersed inside the continuous matrixof fluoroelastomer, essentially in the form of primary particles.

Although the invention has been described in conjunction with specificembodiments, it is evident that many alternatives and variations will beapparent to those skilled in the art in light of the foregoingdescription. Accordingly, the invention is intended to embrace all ofthe alternatives and variations that fall within the spirit and scope ofthe appended claims. The above references are hereby incorporated byreference.

What is claimed is:
 1. A composite material consisting essentially of afluoroelastomer matrix containing a fluoroplastomer in a dispersedstate, said dispersed fluoroplastomer being homogeneously dispersedsubstantially in the form of primary particles, both the fluoroelastomermatrix and the dispersed fluoroplastomer being obtained by emulsion ordispersion polymerization in an aqueous phase in which thefluoroelastomer is selected from the group consisting of copolymers ofvinylidene fluoride and hexafluoropropylene, terpolymers of vinylidenefluoride, hexafluoropropylene and tetrafluoroethylene; terpolymers ofvinylidene fluoride, tetrafluoroethylene and a perfluoroalkylvinylether; copolymers of vinylidene fluoride and hydropentafluoropropylene,terpolymers of vinylidene fluoride, hydropentafluoropropylene andtetrafluoroethylene and the corresponding polymers containing in thechain a small quantity of trifluorobromethylene and the fluoroplastomeris selected from the group consisting of polytetrafluoroethylene;copolymers of tetrafluoroethylene and a perfluoroalkyl vinyl ether;copolymers of vinylidene fluoride and chlorotrifluoroethylene andcopolymers of tetrafluoroethylene and propylene.
 2. A composite materialaccording to claim 1, in which the fluoroelastomer is a copolymer ofvinylidene fluoride, hexafluoropropylene, or a terpolymer of vinylidinefluoride, hexafluoropropylene and tetrafluoroethylene.
 3. A compositematerial according to claim 1, in which the fluoroplastomer ispolytetrafluoroethylene.
 4. A composite material consisting essentiallyof a fluoroplastomer matrix containing a fluoroelastomer in a dispersedstate, said fluoroelastomer being uniformly distributed inside thecoagulated flocks of the fluoroplastomer, both the fluoroplastomermatrix and the dispersed fluoroelastomer being obtained by emulsion ordispersion polymerization in an aqueous phase characterized in that, insaid aqueous phases, the single components are in the form ofdispersions in which the particles have a surface charge of the samesign and a "zeta potential" (in absolute value) between 10 and 11 mV,and the ratio (in absolute value) between "zeta protentials" of theparticles of each dispersion is between 0.2 and
 5. 5. A compositematerial according to claim 4, in which the fluoroplastomer is selectedfrom the group consisting of polytetrafluoroethylene; copolymers oftetrafluoroethylene and a perfluoroalkylvinyl ether; copolymers ofvinylidene fluoride and chlorotrifluoroethylene and copolymers oftetrafluoroethylene and propylene.
 6. A composite material according toclaim 4 or claim 5 in which the fluoroelastomer is selected from thegroup consisting of copolymers of vinylidene fluoride andhexafluoropropylene, terpolymers of vinylidene fluoride,hexafluoropropylene and tetrafluoroethylene; terpolymers of vinylidenefluoride, tetrafluoroethylene and a perfluoroalkylvinyl ether,copolymers of vinylidene fluoride and hydropentafluoropropylene,terpolymers of vinylidene fluoride, hydropentafluoropropylene andtetrafluoroethylene and the corresponding polymers containing in thechain a small quantity of trifluorobromoethylene.
 7. A compositematerial according to claim 4, in which the fluoroplastomer ispolytetrafluoroethylene.
 8. A composite material according to claim 4,in which the fluoroelastomer is a copolymer of vinylidene fluoride andhexafluoropropylene or a terpolymer of vinylidene fluoride,hexafluoropropylene and tetrafluoroethylene.
 9. A composite materialaccording to claim 7, in which the fluoroelastomer is a copolymer ofvinylidene fluoride and hexafluoropropylene or a terpolymer ofvinylidene fluoride, hexafluoropropylene and tetrafluoroethylene.
 10. Acomposite material according to claim 1, wherein at least about 90% ofthe fluoroplastomers are dispersed as primary particles.
 11. A compositematerial according to claim 1, in which the fluoroelastomer is acopolymer of vinylidene fluoride and hexafluoropropylene and thefluoroplastomer is polytetrafluoroethylene.